“Self destruct” eFuse added to kernel which gets triggered if an older bootloader is used. This means Fire TVs that update to stock 51.1.4.1_user_514013920 can never be downgraded, even if a method to root them is discovered.

As of October 2014, some Fire TVs began shipping with a different stock recovery then originally came with the listed software versions. This new stock recovery blocks downgrading. The recovery image linked here is the old version which does not block recovery.

Used for restoring a Fire TV with ClockworkMod back to stock, or for downgrading a Fire TV that blocks downgrades.